1. Field of the Invention
The disclosure relates to a composite, and in particular relates to a method of dispersing a carbon nanotube in the composite.
2. Description of the Related Art
Plastic products, having flexibility and light weight, are widely applied in the household industry. Because the thermoplastic material is inherently insulative, the electrostatic charges may accumulate on the thermoplastic material by rubbing of the thermoplastic material's surface. The electrostatic charges will disturb processes, e.g. degrade a roll-to-roll property of thermoplastic material films or adhere to the thermoplastic material films, attracting dust or dirt to reduce yield of production lines, interfere with or damage electronic equipments, or cause sparks and bursting.
3C electronic products are being developed to have a light weight, and be thin, short, and small, such that development is towards high densities and high frequencies. The electrostatic disturbance and electromagnetic compatibility (EMC) problem of small and high dense devices need to be overcome. A metal shell may shield the electromagnetic wave and achieve anti-electrostatic effect; however, metal is difficult to process due to its high density. The metal shell is therefore replaced by a coating of a conductive paint, sputtering of a metal layer, or electroless plating of a metal layer on the shell; however, these methods of substitution still have problems such as high costs, complicated processes, and environmental pollution. In addition, the metal layer on the shell easily peels to lose its effect.
For application in electronic products, the electrostatic problem of the thermoplastic material can be solved by blending an anti-electrostatic/conductive material into the thermoplastic material. The blend is then extrusion or injection molded to form a composite having anti-electrostatic (109 to 1012 ohm/□), electrostatic discharge (106-109 ohm/□), conductivity (≦106 ohm/□), and electromagnetic interference (≦104 ohm/□) functions.
For averagely dispersing the anti-electrostatic material into the thermoplastic material, an additive and a carrier are mixed, melted, and pelletized to form a master batch. The master batch and the thermoplastic material are then mixed and melted, such that the additive and the thermoplastic material have good compatibility due to wettability and dispersity of the carrier. The current mainstream anti-electrostatic material is ester, amine, and organic salt, which may adsorb moisture in air to form a conductive aqueous layer on its surface. As such, a product must be put in a circumstance with moisture for several days for anti-electrostatic effect, which may easily fail due to insufficient moisture. The anti-electrostatic material easily migrates to the surface of the product by heating the product. In addition, the anti-electrostatic effect of the product is easily reduced or disappears by stretching the product.
If carbon black is adopted as an anti-electrostatic/conductive material averagely dispersed in the thermoplastic material, the carbon will contact or separate by a short distance (usually less than 2 nm) to form a conductive path. However, the carbon black amount needs to meet the requirement of a high percolation threshold. For example, the carbon black serving as the anti-electrostatic material should be greater than 5 wt % of the composite, and the carbon black serving as the conductive material should be greater than 20 wt % of the composite, respectively. A high amount of the carbon black will influence the processibility and mechanical properties of the composite, and depart carbon to pollute the product and influence the thermoplastic material appearance after rubbing of the composite surface. The electric properties of the composite added with the carbon black is still influenced by stretching. There is no conventional composite, wherein its anti-electrostatic property is held constant, even after being stretched for four or more times in size.
A carbon nanotube (CNT) having excellent electric and mechanical properties is suitable to serve as a conductive filler or a strengthening material. However, the carbon nanotube surface is smooth and chemical inert, such that the carbon nanotube and the thermoplastic material have poor compatibility. In addition, the carbon nanotubes have high length/diameter ratio, such that the carbon nanotubes easily attract or tangle to each other due to strong Van der Walls force therebetween. As such, the carbon nanotube cannot be dispersed in the thermoplastic material. Furthermore, the carbon nanotube is inherently light weight, occupies a lot of space, and is easily blown, thereby increasing the trouble in handling and storing the carbon nanotube. Accordingly, the carbon nanotube is difficult to be directly processed and applied.
For averagely dispersing the carbon nanotube in the thermoplastic material, the carbon nanotube can be modified as below. In the first method, the carbon nanotube surface can be chemical modified by a strong acid such as hydrochloric acid or nitric acid, thereby forming carbonic acid and the likes on the carbon nanotube surface. Thereafter, the carbonic acid and the likes can be further grafted or modified. On the other hand, the carbon nanotube surface can be directly modified or grafted by a radical reaction, thereby improving the activity of the inert carbon nanotube surface. Thereafter, the chemical modified carbon nanotube is blended with the thermoplastic material. The chemical modification often damage the carbon nanotube wall, thereby reducing the conductivity of the carbon nanotube. In the second method, or so-called in-situ polymerization method, the monomer and the carbon nanotube are averagely mixed in a solution. The monomer is then polymerized, thereby dispersing the carbon nanotube in the polymer polymerized from the monomer. In the third method, or so-called solution process the, the carbon nanotube and the polymer solution are averagely mixed. A solid is then obtained by re-precipitation or removing the solvent of the mixture. This method can averagely disperse the carbon nanotube in the thermoplastic material, however, it is not suitable for mass production due to its complicated processes, high cost and toxicity of the solvent, and solubility limitations of the polymer in the solvent.
Accordingly, a method of efficiently dispersing the carbon nanotube in the thermoplastic material without damaging the carbon nanotube surface is still called-for.